A very widely used method in the art for the stabilization of olefin polymers consists of mixing the polymers with the stabilizers and extruding the mixtures thus obtained. In this manner one obtains pellets in which the stabilizers are dispersed relatively homogeneously, providing a good stability to the pellets which in turn can be transformed directly into finished products.
The above mentioned techniques for pelletizing by way of extrusion are particularly advantageous in the case of polymers obtained in the form of irregular particles and with low flowability, which consequently must be granulated in order to obtain a commercial product. Ziegler-Natta catalysts supported on magnesium chloride, and polymerization processes based on the use of such catalysts, are establishing themselves in the field of olefin polymerization. Such catalysts are capable of producing polymer particles that because of their high flowability, high bulk density, and absence of extremely fine particles, can be commercialized directly, without having to be transformed into extruded granules. There is a definite need, therefore, for a process that allows one to stabilize the above mentioned particles directly, without requiring subsequent extrusion. It is of particular interest to obtain a stabilizing effect against oxidating phenomena which occur during polymer storage.
One of the solutions proposed in the known art to reach the above mentioned objective consists of adding one or more stabilizers during the polymerization process. This solution is particularly interesting because it would mean that no treatment of any kind would be needed downstream from the polymerization plant. According to U.S. Pat. No. 3,477,991, in order to obtain a stabilized polymer, alkenylphenols are added in an .alpha.-olefins polymerization process carried out with a TiCl.sub.3 based catalyst. However, as shown in published European patent application EP-A-0192987, adding phenol compounds to Ziegler-Natta catalysts supported on magnesium chloride causes unacceptable decreases in stereospecificity, or a yellowing of the polymer. In order to overcome those limitations, the European patent application suggests the addition of HALS (hindered amine light stabilizers) during the polymerization phase. In this manner one can obtain a stabilizing effect against thermal oxidation.
According to published European patent application EP-A-0254 348, one can obtain good polyolefin stabilization, particularly against degradation phenomena which occur during the handling of polyolefins in the molten state by adding phosphites and phosphonites during polymerization. Moreover according to published European patent applications 350444 and 351360, various combinations of HALS, phosphorous esters, and phenols are also added during polymerization in order to obtain a stabilizing effect against thermal oxidation.
The above mentioned documents show that the addition of HALS and phosphorus esters does not interfere much with the activity of the Ziegler-Natta catalysts supported on magnesium chloride, and allows one to obtain polymers with no yellowing effects.
Moreover, the above mentioned published European patent application 351 360 shows that, thanks to the addition of the phosphorus compound, one can also add moderate quantities of phenolic stabilizers without yellowing of the polymer.
However, it must be pointed out that all the above mentioned solutions do not solve the problem of eliminating the damaging effects caused by the addition of phenolic stabilizers in a polymerization carried out with supported catalysts, but require the use of other stabilizers, partially or totally substituting the phenolic stabilizers.
On the other hand, it is well known that in practice, the phenolic stabilizers are the only stabilizers capable of conferring to the polyolefins a high resistance to thermal oxidation phenomena, and to oxidation and discoloration phenomena which occur during the storing of the polymer.
Moreover, by adding to the polyolefins HALS (typical stabilizers against light induced oxidation), phosphorus esters (typical process stabilizers used to avoid polymer degradation during its handling in the molten state), and phenolic stabilizers, one limits the final use of the polyolefins, since the user does not have the possibility of choosing a stabilizing formulation aimed at a specific end-use both in qualitative and quantitative terms.